The invention relates to bio-compatible surfaces, in particular to non-thrombogenic surfaces, and is particularly concerned with the provision of vascular catheters and cardiac assist devices incorporating such non-thrombogenic surfaces.
That blood remains fluid in blood vessels is partly due to the fact that normal vascular endothelium does not promote blood coagulation. Foreign surfaces, both endogenous and exogenous, promote clotting in varying degrees. The degree of clotting appears to depend, inter alia, on the surface electrical charge and the property of wettability of the surface. The inactive coagulation factors, factor XII (Hageman factor) and factor XI, are activated by contact with foreign surfaces, and then the coagulation process is initiated. This process may be initiated in the intravascular space by such foreign surfaces as tumor cells, disrupted villi as in accidental ante-partum hemorrhage, endothelial cells damaged by trauma and infarction, or by the introduction of foreign bodies such as catheters or the like.
Some silicone surfaces and a number of plastics materials have a weak effect in promoting blood clotting. However, even a weak thrombogenic effect makes such materials unacceptable for use in situations were clots which may be formed could block the flow of blood to vital organs such as the brain or heart. Non-thrombogenic properties are particularly desirable for intravascular prostheses, catheters and extracorporeal equipment such as heart-lung machines and the like.
Attempts have been made to impart non-thrombogenic properties by adding to, or otherwise modifying, the surface of various polymeric materials but such attempts have not been as successful as desired. In particular, such materials have not totally alleviated the problem associated with catheters and the like so that periodic replacement, for example during long term treatment, is necessary. Repeated replacement may cause extensive damage to blood vessels including partial or total collapse of the vessel or may cause scar tissue to form at the site of introduction thus making subsequent procedures more difficult.